1. Field of the Invention
The present invention relates to a main element of a protector device and its fabrication method which returns itself to its non-conductive state in a very short time after conversion to its conductive state by a surge including thunder.
2. Related Background Art
A surge protector device including an arrester is very important device to protect various electronic devices from surge including thunder. The surge protector device is a general name of devices which are used in order to protect other electronic devices from excess voltage, that is surge. An arrester is used to protect other electronic devices from thunder, that is extremely high voltage and large current. The arrester is one of the surge protector devices. The term of xe2x80x9cprotector devicexe2x80x9d is used here to indicate devices which are used in order to protect other electronic devices from excess voltage or excess current. However excess voltage is not limited to extremely high voltage such as thunder but includes low voltage if it is excess to a specified voltage.
A glass-tube type arrester has been used. It contains special gas between two electrodes in a glass tube. It is non-conductive unless surge is induced. When surge or thunder is induced, discharge starts and the gas between the electrodes changes to conductive. Current flows through the arrester and, it is lead to the earth. Discharge does not stop immediately after surge is removed. The arrester cannot protect other electronic devices from continuous current or next attack by surge or thunder. There are serious problems which a glass-tube and other type protector devices have which have been used. One of it is that a protector device must change from its resistive state to a conductive state in a very short time such as 0.03 xcexcsec. when it is attacked by surge. Another problem is that a protector device should return from its conductive state to its resistive state when surge is removed.
In order to solve these problems in the prior art an arrester was proposed (Japanese Patent 118361, 1995 xe2x80x9cMolybdenum arresterxe2x80x9d by Seita Ohmori). It used a plural of molybdenum bars whose surface was oxidized. The arrester will be called here as a xe2x80x9cmolybdenum arresterxe2x80x9d.
The molybdenum arrester leads current to the earth in a very short time when surge or thunder is induced. That is, it changes from non-conductive state to conductive state very quickly by breakdown of the oxide formed on the molybdenum bar. Moreover, it returns from conductive state to non-conductive state when surge or thunder is removed because molybdenum is oxidized quickly if it is in oxidizing atmosphere. The molybdenum arrester is very useful and economically efficient because it repeats change of the state automatically.
It is possible to use metals other than molybdenum in a protector device which functions with same principle as the molybdenum arrester. Tantalum, chromium and aluminum are included in such metals.
There is a serious problem in a protector device of the prior art which comes from the fact that the protector device uses a plurality of bars which have high resistive films on their surfaces. FIG. 1 shows schematically the protector (10) of the prior art which is called the molybdenum arrestor proposed by Ohmori (Japanese Patent 118361, 1995 Molybdenum arresterxe2x80x9d).
The arrestor (10) includes two molybdenum bars (11) which have high resistive oxide films (12) on their surfaces and electrodes (13). The arrestor (10) uses breakdown phenomenon at the interface between the high resistive films (12). A breakdown voltage depends largely on microscopic structure of the interface. That is, as shown in FIG. 2, the high resistive films (12) on the two molybdenum bars contact point by point microscopically although they seem to contact line by line or surface by surface macroscopically. It is difficult to control the microscopic structure at the interface during fabrication process. Breakdown occurs at a point where largest electric field is applied by a surge. A breakdown voltage also depends on force induced to the interface. Therefore, it is impossible to design and fabricate the arrestor of the prior art with a precisely controlled breakdown voltage. The problem cannot be solved as far as a protector device uses breakdown phenomenon at the interface between two surfaces.
It is desirable, therefore, to provide a surge protector device which does not use breakdown phenomenon at the interface between two surfaces.
The present invention is directed to a main element of a surge protector device and its fabrication method which uses breakdown phenomenon of a single high resistive film. A breakdown voltage and a place where breakdown occurs can be precisely controlled. The surge protector device changes from its non-conductive state to conductive state very quickly when a surge is induced, and returns quickly to the non-conductive state when a surge is removed if the element is surrounded by oxidizing agent.
The main element of the surge protector device of the present invention has a single high resistive film on a single metal bar. The high resistive film has a part or parts where electric field concentrates when a surge induced. A breakdown voltage can be controlled precisely by controlling a size including a thickness of the high resistive film of the part. The part is called a fuse part here. It is possible, therefore, to form a plurality of fuse parts such that they have the same breakdown voltage or different breakdown voltages.
A preferred metal is molybdenum although other metals can be used.
The surge protector device of the present invention is fabricated by a method which includes following steps. At the first step, a metal bar is prepared and washed with a suitable solvent followed by etching of the surface. At the second step, the metal bar is pre-heated in an atmosphere which does not contain oxygen in order to drive impurities from the bar. At the third step, an insulating film is formed in an atmosphere which contains no oxygen. At the fourth step, the insulating film is patterned to expose the main surface of the metal bar in the areas where two pad parts and at least one fuse part will be formed. In general, a size of the fuse part is much smaller than the pad parts. At the fifth step, the metal bar is oxidized in the areas which were exposed at the fourth step. A high resistive film is formed by this oxidation. At the sixth step, another insulating film is formed on the entire surface of the metal bar. The previously formed insulating film and an oxide film are covered by the new insulating film. At the seventh step, the new insulating film is patterned to expose the high resistive film in the area of the fuse part. At the eighth step, the high resistive film is etched to a predetermined thickness in the fuse part. Then the new insulating film is removed from the pad areas. At the ninth step, electrodes are formed on the high resistive film in the pad areas.